First, over 100 Years of history.
Second, it's hard to store AC.
Third, the AC frequency varies as the engine speed. A varying frequency is problematic for many AC devices.
Alternators have many advantages over generators in automotive applications, but automotive alternators had to wait until the development of small, dependable and relatively cheap semi-conductor rectifiers before being applied to automotive use.
AC in vehicles is not a new or unused idea. 400 Hz AC has been used in aircraft for well over 50 years, probably over 70 years. However the economics of aircraft are much different than the economics of automobiles. The importance of saving weight in aircraft design is vastly more important than weight saving in automotive design.
With increasing emphasis on fuel economy, weight saving is gaining in importance in automotive design. If the economic importance of weight saving continues to increase, a point may be reached when the proven technology of mobile 400 Hz AC power may become an automotive solution.
A note; When 400 Hz aircraft systems were introduced, cheap, small and dependable rectifiers had yet to be developed. A DC generator was much heavier than a 400 Hz alternator of the same capacity. (As a general rule, higher frequency means less weight.)
We may still need stored energy for engine starting.
Bill
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"Why not the best?"
Jimmy Carter
Even in the largest aircraft there is a DC system, or two, with a battery used for starting the aircraft engines and emergency power. 400 Hz is available at high power only after the engines starts. In aircraft the voltage is 'called' 28 VDC which actually is the same voltage as large trucks which 'call' it 24 VDC. Some small single engine aircraft use 12 VDC just like cars. If you look at an alternator in a piston engine aircraft, it looks just like a car or truck alternator because it's basically the same alternator just built with better bearings and components.
Yeah the alternator is generating AC but it is being turned into DC before it leaves the alternator case, and its bad enough having the ripple on that DC when it comes to powering the massive amounts of ECU's or electronic modules in todays cars and trucks, having AC running throughout would require more energy robbing circuits for converting and controlling the AC. And like others mentioned battery's are DC. There would be no savings in manufacturing or energy wise to use AC.
Just think of the simple things like seat and window motors with AC all reversible motors would likely have to be 3 phase, the switch and wiring becomes more complex the motor itself more costly. And what about the AC em fields that would induce AC into wiring you don't need that sort of interference in? Its a no brainer to keep what we have in the auto industry. Using AC in cars would be like manufacturing dinner plates from titanium.
Hard to see DC going away, if anything there is more and more of it with the hybridization and electrification of propulsion systems. What I do find strange is how long the 14 VDC systems have hung on. Seems like it was nearly 20 years ago that all the papers and magazine articles were touting the coming of 42 VDC systems. With the proliferation of electrically heated seats and steering wheels, electric power steering, start/stop systems, etc. you would think the smaller gauge wiring requirements at the higher voltage would have taken off long ago. A lot of vehicles have high voltage subsystems, past time to drop the low voltage systems.
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I would think that's tied to a combination of factors including volumetric efficiency of the battery, power requirements of the subsystems, and alternator volumetric and generation efficiency.
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I suspect that 42V was stillborn because of conflicting/overlapping regulatory fiefdoms.
The last time I looked, which was quite a while ago, it was illegal to sell, and perhaps possess or use, ANYTHING containing a DC voltage greater than 30 volts within the City of Los Angeles unless it bore a UL listing. There are probably similar gotchas in other city codes, but LA is known to be particularly picky.
UL doesn't list automobiles, and it would cost much money and time to cause them to develop such a standard, horrendous amounts to gain a listing for a particular model, not to mention the cost to do it all over after every running line change.
They probably could have grandfathered 24/28V systems with no hassle.
US Navy prohibits anything in excess of 30 volts where it could be touched, such as a broken/cracked pushbutton or changing a broken light bulb. The theory is that the human body resistance can drop as low as 300 ohms; tired, out of shape, sweaty, and 30 volts could result in 0.1 amps of current which is considered lethal. Also Ford/Lincoln briefly offered a high powered window defroster that used a dedicated 100 volt alternator. After several people were hurt, the system quietly went away. But why no 24/28 volt remains a mystery.
At one time there was an issue with incandescent lamps. As the rated voltage rises, for a given wattage lamp, the filament diameter decreases. Many years ago, there was a shift towards 24 Volt systems for large trucks. It was becoming difficult to start large diesel engines with 12 Volts and 24 Volts was the answer. It was soon found to be extremely difficult to keep lamps in the trucks. The thinner filaments could not withstand normal road and engine vibration and frequent lamp burnout was the result.
Hence the series parallel switches and 12/24 Volt systems that served well for at least 50 years.
Lamp technology must have improved as today, straight 24 Volt systems are common on equipment such as front end loaders and there does not sem to be an issue with lamp burnout.
Bill
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"Why not the best?"
Jimmy Carter
It was probably over 60 years ago when the light bulb issue arose with 24 Volt systems. The 12/24 systems with the series parallel switches were standard when I first came in touch with large truck systems. Old timers recalled light buld issues with straight 24 Volt systems.
As time passed interoperability most likely became the driving force to stay with the 12:24 systems. Many drivers are not expected to wait while a trailer is loaded or unloaded. They pull into a yard, drop a trailer, hook up to another trailer and go.
Can you imagine the logistics of working with a mix of 12 Volt and 24 Volt trucks and a mix of 12 Volt and 24 Volt trailers?
On the other hand, 20 years ago I was running into straight 24 Volt front end loaders from Caterpillar. It probably helped that the head lights and work lights were rubber mounted but there were no longer lamp burn-out issues.
Bill
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"Why not the best?"
Jimmy Carter
Greglocock - LED lights in automobiles have only appeared in the last couple of years and very few for the highest power draw (headlights). I'm not aware of any fundamental improvements in the primary lead acid batteries still predominantly in use in the last 20 years. Alternators and starter motors may have improved, I don't know.
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
It would not be totally out of the realm of possibility to put an inverter system in the circuit. Although I have had durability issues with inverters outside of a closed environment.
